An airplane begins it's descent to and from a height of 35,000 feet above ground level. the airplane's height decreases by about 4,000 feet every 3 minutes. rounded to the nearest minute, in approximately how many minutes will the plane land ?

A formula for the height of the plane as a function of time (t, in minutes) would be 

h(t) = 35000 ft - (4000 ft / 3 min)(t).

When will the plane land?  Let h=0 and solve for t:  35000 = (4000/3)t
                            35000 ft
Solving for t, t = = 26 minutes (approx).
                           (4000/3) (ft/min)

The answer will be X = 24, -24